SquirrelMail is a PHP-based Web email client. It includes built-in pure PHP support for IMAP and SMTP, and renders all pages in pure HTML 4.0 for maximum compatibility across browsers. It has strong MIME support and a flexible plugin system.

Java interfaces and sample implementations for controlling a model railroad layout from a personal computer. JMRI is intended as a jumping-off point for hobbyists to build their own layout controls. Includes the DecoderPro and PanelPro applications.

Areca-Backup is a file backup software that supports incremental, image and delta backup on local drives or FTP servers. Areca-Backup also allows you to browse your backups and navigate among different version of the files contained in your archives.

Viking is a free/open source program to manage GPS data. You can import, plot and create tracks, routes and waypoints, show OSM, Bing Aerial and other maps, geotag images, see real-time GPS position (not in Windows), make maps using Mapnik (not in Windows), control items, etc.
It is written in mostly in C (with some C++) & the GTK+2 toolkit.